1. Field of the Invention
The present invention relates to optical elements and particularly relates to an optical element suppressing the generation of harmful light causing flares or ghosts.
2. Description of the Related Art
For optical elements such as lenses, various techniques have been proposed for the purpose of suppressing the generation of harmful light that causes flares or ghosts to obtain high-quality, high-performance optical systems. The techniques are classified into two: (1) a technique for reducing reflection by increasing the light transmittance of an optically effective area and (2) a technique for reducing reflection by increasing the light absorbance of an optically ineffective area.
The term “optically effective area” as used herein refers to an area through which light passes. The term “optically ineffective area” as used herein refers to an area through which no light passes. In the case of installing a plurality of lenses such as camera lenses in a lens barrel, although the lenses individually serve as optically effective areas, some of the lenses are likely to serve as optically ineffective areas because no light passes therethrough depending on the sizes and positions of other lenses.
As an example of the technique specified in Item (1), the following technique is widely used: a technique for providing a dielectric coating on an optically effective area by a vacuum vapor deposition process, a sputtering process, or a similar process. Known is an antireflective element using a sub-wavelength structure (SWS) not greater than a wavelength used instead of such a dielectric coating.
Japanese Patent application Laid-Open No. 2006-053220 discloses that a curved surface for forming a member has an antireflective area having periodic micro-irregularities arranged at a pitch not greater than the wavelength of a light beam to be prevented from being reflected. The antireflective area is capable of obtaining antireflection performance excellent in wavelength characteristic and incident angle characteristic as compared to conventional antireflective structures including dielectric coatings.
As an example of the technique specified in Item (2), the following technique is widely used: a technique for reducing internal reflection by forming a coating substantially opaque at wavelengths used on a side end portion (commonly called an edge portion) of a lens. The following paint is usually used to form the coating: an internal reflection-preventing paint prepared by kneading a resin component with a light-absorbing material such as coal tar, coal tar pitch, a black pigment, a black dye, or carbon black. Japanese Patent application Publication No. 47-32418 discloses that internal reflection is reduced using several dyes including a black dye in combination with pitch, carbon black, and an epoxy resin. Japanese Patent application Laid-Open No. 55-155064 discloses that a good internal reflection-reducing effect and the mechanical strength of a coating are secured using a halogenated epoxy resin in combination with coal tar and coal tar pitch.
In addition to the materials disclosed in the above-reference patent documents, epoxy resins and modified epoxy resins are widely used as resin components of internal reflection-preventing paints. This is because cured products having high strength and excellent weather resistance can be obtained using the epoxy resins in combination with appropriate curing agents depending on intended purposes. The epoxy resins are particularly appropriate for the purpose of forming coatings required to have long-term durability. Usually, an epoxy resin or a mixture of the epoxy resin and an additive for imparting a function to the epoxy resin is referred to as a base compound or a base resin with respect to a curing agent. Makers usually designate curing agents used in combination with base compounds depending on intended purposes.
In recent years, coatings have been sometimes subjected to manufacturing processes at higher temperatures as compared to conventional processes for the purpose of improving the performance of optical element. This increases the load on the coatings to negatively affect the function and appearance of the coatings during long-term use.
In the thermal analysis of a cured product of an antireflective paint prepared by mixing an epoxy resin with a black pigment, a common curing agent, and the like, the heating of the cured product to temperatures higher than those used in common manufacturing processes may cause a reduction in the weight of the cured product and may confirm an increase in the glass transition temperature (Tg) of the cured product. These phenomena suggest the cured product is degraded and is reduced in flexibility by thermal decomposition and oxidation. Thermal decomposition and oxidation possibly cause phenomena such as the formation of cracks and/or voids in actual opaque coatings and the removal of the coatings from bases of optical elements by thermal stress. Usually, these phenomena result in the whitening of appearance and the easiness of moisture penetration.
The present invention has been made against such a background and provides an optical element including an optically ineffective area partly or entirely coated with a coating which contains a cured product having improved heat resistance and flexibility and which is opaque at wavelengths used.